The present invention relates to RF (radio frequency) combining apparatus, and more particularly to a diplexer that can be used in a double conversion TV tuner and also to a multiplexer.
In television tuners that tune both VHF and UHF signals of the type described in U.S. Pat. No. 4,408,348 filed in the name of G. E. Theriault, signals in the respective band are first filtered by respective tunable filters and then applied to respective RF amplifiers. The output signal of the VHF amplifier is passed through an LPF (low pass filter) having a cutoff frequency of just beyond 402 MHz (the upper limit of cable channel W+17). The output signal of the UHF amplifier is passed through an HPF (high pass filter) having a cutoff frequency of just below 470 MHz (the lower limit of channel 14). The output signals of both filters are combined and the combined signal is then applied to the signal terminal of the unbalanced input port of a balun. The signals developed at the balanced output port of the balun are applied to the input of a balanced mixer.
The purpose of the LPF and HPF is to inhibit reception of signals in one band when channels in the other band are selected for reception. However, because the cutoff frequencies of the filters are relatively close and the filters have a gradual cutoff characteristics, and further because each filter may have spurious resonances in the other filter's passband, the filters may undesirably interact, e.g., the LPF may cause attenuation of UHF signals and the HPF may cause attenuation of VHF signals.
Mechanical or electronic switches can be used to select between the filter outputs to prevent interaction. Mechanical switches have limited lifetimes. Electronic switches, such as series connected switching diodes, have inherent capacitance that can resonate with stray inductances, which can cause undesirable attenuation of the selected signals particularly the UHF signals. In addition, to provide the desired degree of decoupling, a series-connected and a shunt-connected switching element often has to be used for each signal to be switched. Thus if two signals have to be switched, four switching elements (e.g., diodes) may be required which is costly.
It is therefore desirable to provide a diplexing apparatus for selectively coupling one or two RF signals to a common output without the problems of the above-described combining apparatus. It is also desirable to provide a multiplexing apparatus for selectively coupling one RF signal to one of two outputs with similar characteristics.